What Causes Endometriosis
In truth, nobody is really sure what causes endometriosis. As fast as one theory attempts to explain the occurrence of endometriosis, a special set of circumstances is demonstrated in a particular group of patients that negates that particular theory. The most widely accepted theories include:
1. Retrograde menstruation and implantation
This is the most classic theory. It suggests that blood and fragments of endometrial tissue travel up the fallopian tubes at menstruation instead of flowing down into the vagina. These tissue fragments then are able to implant within the pelvis and become endometriosis.
This theory is attractive in that it has been shown to be true in animal models by injecting samples of menstrual blood into the abdominal cavity and looking for evidence of seeding later. However, like all the theories associated with endometriosis, it does not hold true for all patients.
Endometriosis has been demonstrated in patients who have had tubal ligations many years before. In this circumstance, it is impossible for retrograde menstruation to occur. Also, this theory fails to explain the diverse sites in which endometriosis can occur. Retrograde menstruation cannot explain endometriosis occurring in an abdominal wound or in the umbilicus, for example. It therefore may provide some of the answer as to why endometriosis occurs, but it is clearly not the whole reason.
2. Spread of Endometrial Tissue by the Blood or Lymphatic Channels
It has been suggested by some authors that fragments of endometrial tissue are carried by the veins or lymphatics (channels in the body which parallel the bloodstream and are responsible for carrying the clear serous fluid which makes up most of our body tissue).
This theory may have some part to play in the transmission and spread of endometriosis and may explain the very rare sites, such as lung, which are far from the pelvis, where endometriosis can occur. However, most workers in the field feel that this explanation is insufficient to explain how endometriosis really occurs.
3. Metaplasia, The Immune System and Stem Cells
The ability of cells to undergo metaplasia provides the basis of the most plausible theory as to why endometriosis occurs. Metaplasia is a process in the body whereby one adult cell type can undergo transformation to become another adult cell type. For example, a cell may change its shape and specialised function from being a long, thin cell, which produces mucous to being a round, flat cell which protects the surface of the body upon which it sits.
This sort of metaplasia occurs every day in the cervix. Most of the specialised types of cells and tissue occurring in the pelvis are derived from the same primitive cell line when a baby is forming in utero. Therefore, the ability exists right throughout your life for one specialised cell type to change into another specialised cell type, provided it comes from the same primitive tissue cell line and it is given the appropriate set of stimuli. This forms the basis of how endometriosis is thought to occur.
Specialised adult types of cells that line the pelvis, such as the pelvic peritoneum, can therefore change into endometriosis type cells, provided the correct set of stimuli occur. We are unsure as to what these stimuli are. Current theories suggest that patients who develop endometriosis may have an imbalance of hormones and chemicals released into the pelvic cavity by the ovaries.
These hormones and chemicals, such as oestrogen, progesterone and prostaglandins, may provide the stimuli to change pelvic cells into endometriosis cells. Conditions where there is a very low output of hormone from the ovary, such as pregnancy, lactation or anti-endometriosis drug therapy, can therefore decrease the stimuli to the pelvic cells and decrease the rate of change to endometriosis type cells. Other factors associated with regular monthly ovulation may modify this response.
There are clearly a group of susceptible patients who develop endometriosis, even though their ovulation may not be much different from other patients who do not develop endometriosis. There is exciting new research work into the role of stem cells and whether they undergo special transformations to become endometriosis.
Why will patients respond differently to endometriosis treatment?
We do not know why some patients do and some patients don’t respond to these stimuli. The immune system may have an important part to play in preventing the development of endometriosis or allowing it to occur.
It has been well demonstrated that patients with endometriosis have altered immune function. It is likely that the immune system alternately allows endometriosis to grow and then perhaps contributes to its dying off so causing small areas of scar tissue in the body. Immunology is still a relatively poorly understood area of medicine and the protective role it may play in the development of endometriosis is still poorly understood.
Further research is clearly required into the aetiology of endometriosis and this is currently being performed in a number of centres around the world.
